1. Field of the Invention
The present invention relates generally to a control scheme. More particularly the present invention relates to a method and apparatus for reducing a shaft power required to drive a multistage turbocompressor by selectively manipulating throttle valves at the compressor stages' inlets while simultaneously protecting the compressor stages from surge.
2. Background Art
During some modes of operation a load imposed by the process on a single- or multistage compressor may exceed a maximum power available from the driver or drivers. Compressor shutdown may be required to avoid damage to the driver. Shutdown is to be avoided due to its inherent production loss.
A known method to avoid shutdown while still protecting the driver from damage reduces the load on the train by throttling the inlet flow using an inlet throttle valve on each stage of compression.
The present-day scheme of protection calls for reducing the opening of the inlet throttle valves, when present. The anticipated result is a reduction of flow through each of the compressor stages, and a consequent reduction in power consumed by compressor train.
Compressor surge is an unstable operating condition that is to be avoided. Modern control systems provide antisurge protection by calculating an operating point of the compressor and determining a proximity of the operating point to the compressor's surge limit. Antisurge control is explained in the Compressor Controls Series 5 Antisurge Control Application Manual, Publication UM5411 rev. 2.8.0 Dec. 2007, herein incorporated in its entirety by reference.
A surge control line is defined by providing a safety margin to the surge limit. When the compressor's operating point approaches the surge control line, a recycle, or antisurge, valve plumbed in parallel with the compressor is opened to provide sufficient flow to the compressor to keep it safe from surge.
Throttling the inlet flow of a turbocompressor stage operating at or near its surge control line causes that stage's operating point to be driven nearer to surge. When the antisurge control system is actively manipulating the antisurge valve to protect its compressor stage from surge, closing the inlet throttling valve will cause the control system to increase the opening of the antisurge valve to compensate for the reduction of the inlet flow rate. Thus no reduction of shaft power is realized.
There is, therefore, a need for an improved control strategy for the startup of turbocompressors to reduce the loading of the compressor while maintaining the compressor flow out of the unstable, surge region.